1. Field of the Invention
The present invention relates to a digital protective relay, and more particularly to a decoding unit for decoding a time reference signal in the digital protective relay.
2. Related Background Art
In the digital protective relay, for the purpose of judging whether a breaker in an electric power system is tripped by detecting a change in state of the power system and adding an exact time of occurrence to data about the change in state and thus recording the data, a time reference signal is inputted from outside of the digital protective relay, and a time of an internal timer is synchronized with this time reference signal. An IRIG signal may be given for use by way of one example of the time reference signal. The IRIG signal serves to transmit time data in serial codes and also a precise timing of updating the time at a timing of a head of frame as well as a rise of a carrier the IRIG signal is defined as an amplitude modulation signal, wherein a level ratio when an amplitude is large and small is specified such as 3.3:1 (=10:3), however, there is no definition in terms of a specific voltage level and waveform as well.
Herein, let VH be a level when the amplitude is large, and VL be a level when small. Based on when changed from the level VL to the level VH, a code is [1] when a continuous time ratio of the level VH to the level VL is 5:5, and the code is [0] when at a ratio of 2:8. When the continuous time ratio is 8: 2, that is represented by a marker code [P] as a reference of a time frame, and there becomes a head of the time frame when the code [P] continues twice.
There are some categories of the IRIG signal according to a time scale, however, a carrier signal with a frequency of 1 kHz and an IRIG-B signal with a time frame being 1 sec are comparatively widely used. FIG. 6 shows a waveform image of the code [0] of the IRIG-B signal. FIG. 7 shows a waveform image of the code [1]. FIG. 8 shows a waveform of the code [P].
From what has been described above, there must be a necessity for judging the levels VH and VL and discriminating and extracting binary data of code[0]/[1] on the basis of a continuous time of the level VH or VL in order to decode the time data from the IRIG signal.
FIG. 9 is a block diagram showing processes starting from taking in the IRIG signal to decoding in the conventional digital protective relay. FIG. 10 is a graph showing relations among signals T1xcx9cT6 in the respective portions in FIG. 9. An IRIG signal T1 is received by an insulating device 1 and transmitted through a full-wave rectifier 2, thereby obtaining a signal T2.
A smoothing circuit 3 smoothes the signal T2, thereby obtaining a signal T3. The signal T3 is inputted to a comparison input terminal of a comparator 91 and compared with a voltage threshold value VTH inputted to a reference input terminal. The voltage threshold value VTH inputted to the reference input terminal is obtained by dividing a voltage of, e.g., 5V with a voltage divider constructed of a resistor R7 and a variable resistance VR. The comparator 91 binarizes the signal T3 inputted to the comparison input terminal depending on a magnitude of the compared result, thereby obtaining a signal T4. A timer 93 is reset and started at a rise of the signal T4 and stopped at a fall of the signal T4, whereby a continuous time when the signal T4 is at the H-level can be measured. A code discriminator 94 discriminates the measured continuous time. Based on two threshold values preset therein, if existing in a short/small region, the code [0] is outputted. If existing in an intermediate region, the code [1] is outputted, and, if existing in a long/large region, the code [P] is outputted.
An output of the code discriminator 94 is stored in a memory 5 and converted into a time signal by software on a CPU 6. A head of the time frame comes with a trigger being a timing when the code [P] continues twice, and a weight of each code with respect to the time is predetermined. Then, a series of time signals can be univocally converted time values.
A problem inherent in the conventional digital protective relay described is that a larger number of processes are needed because of the voltage threshold value VTH in the comparator 91 having to be adjusted with the variable resistance when in the manufacturing process of the digital protective relay, and that a great number of electronic parts are required for decoding, resulting in an increase in cost.
It is a primary object of the present invention to provide a digital protective relay capable of eliminating a necessity for adjusting an input circuit of a time reference signal and reducing costs by decreasing the number of electronic parts.
To accomplish the above object, the digital protective relay according to the present invention is constructed so that a value, into which the time reference signal is A/D converted, is compared with a fixed threshold value by software on a CPU in order to discriminate H- and L-levels from each other, and a time code is obtained by distinguishing between codes [0]. [1] and [P] in accordance with an H-level continuous time on the basis of a timing when changed from the L-level to the H-level. This contrivance eliminates necessities for a comparator, a timer and a timer value judging circuit and for adjusting a voltage of the comparator.
It is possible to A/D convert the time reference signal and execute decoding into a time code by use of a CPU and an A/D converting unit used for a protective relay calculation. With this contrivance, new pieces of hardware required can be omitted, and the costs for the hardware can be reduced.
A peak value is calculated by the software on the CPU from the data into which the time reference signal is A/D converted, and a threshold value for VH and VL is determined based on this peak value. The H- and L-levels are judged based on this threshold value, and the time code can be obtained by distinguishing between the codes [0], [1] and [P] in accordance with the H-level continuous time on the basis of the timing when changed from the L-level to the H-level. An input circuit of the time reference signal shown in FIG. 1, when a voltage level of the time reference signal rises, an output voltage of a smoothing unit 3 when at VL also rises. Hence, if the threshold value for distinguishing between the H- and L-levels is fixed, a voltage of the time reference signal that can be inputted is restricted. According to this contrivance, the threshold value may be taken large when the voltage of the time reference signal is high and taken small when low, whereby a voltage range of the time reference signal can be expanded.
In the digital protective relay constructed such that a peak value is calculated by the software on the CPU from the data into which the time reference signal is A/D converted, the threshold value for VH and VL is determined based on this peak value, the H- and L-levels are judged based on this threshold value, and the time code can be obtained by distinguishing between the codes [0], [1] and [P] in accordance with the H-level continuous time on the basis of the timing when changed from the L-level to the H-level, the peak value can be calculated by A/D converting the time reference signal at a sampling frequency asynchronous to a carrier frequency of the time reference signal. If a greatest common divisor of the sampling frequency and the carrier frequency of the time reference signal is large, it follows that a sampling angle becomes large, and there might be a case where the peak value of the time reference signal can not be detected at a high accuracy. According to the contrivance described above, however, even in such a case, a sampling phase shifts so as to be capable of sampling in the vicinity of the peak value, and it is therefore feasible to properly set the threshold value and enhance a degree of allowance for decoding the time code.